The long-term objective of this project is to investigate the hypothesis that adenosine functions as a negative-feedback regulator of catecholamine-elicited responses in the mammalian heart. One specific aim is to reveal the importance of extracellular (interstitial) adenosine in the antiadrenergic actions of the nucleoside in isolated working hearts. Another specific aim is to investigate the ability of adenosine to modulate intracellular calcium of adult primary cultured ventricular myocytes using an intracellular calcium sensitive fluorescent indicator. Finally, and of primary importance are the studies designed to investigate the membrane biochemical mechanisms by which adenosine, upon interaction with its inhibitory receptor, manifests its antiadrenergic actions in the myocardial cell. For the last series of studies cultured ventricular myocytes and ventricular membranes will be utilized. The knowledge gained from this project should contribute significantly to the physiological characterization of the antiadrenergic action of adenosine in the normal working heart. The new knowledge should also provide insight into the way in which mechanisms regulating myocardial contractile state are altered in the failing and hypoxic heart, such as in conditions of cardiac and coronary artery disease.